The invention relates to a radio antenna arrangement for establishing a radio link with terrestrial radio stations for decimetric or centrimetric waves, having an antenna mounted on an inclined window of a substantially electrically conductive motor vehicle body. A radio antenna arrangement of this kind can advantageously be used for instance for the radio systems in mobile communications (car telephones in the C, D or E network, or for trunk-type radio systems).
If a mounting hole is not to be made in the vehicle body, then frequently a rod-like antenna is mounted in the upper region of the rear window of the vehicle. The antenna element is then secured to the outside of the pane of glass, and the counterweight of the antenna is likewise mounted on the window pane, for instance, or for instance is embodied by the coaxial supply cable, which can be laid entirely in the vehicle interior, and the capacitive input of the antenna signals can take place through the pane.
A disadvantage of such an antenna in accordance with the prior art is that because of the slope of the rear window, the base of the antenna element is located markedly below the edge of the vehicle roof. As a result, at least in its lower region, the antenna element is necessarily masked toward the front by the vehicle body. Moreover, because of the vicinity of conductive parts of the vehicle body, there is always a strong radiation coupling to the body resulting in strong currents therein, which in turn cause an energy release by radiation, thereby greatly affecting the directional diagram.
The characteristic of the horizontal diagrams sought for motor vehicle radio antennas, with the most uniform possible emission in all directions in space, is in practice therefore approximately attained only by means of rotationally symmetrical antenna elements in the center of the roof. With antennas mounted eccentrically or with antennas adhesively mounted on the window pane of the vehicle, the radiation coupling with the vehicle body causes undesirable and sometimes no longer tolerable deformations of the horizontal diagram. These are in particular radiation compensations that cause major, contractions in the horizontal diagram. As a rule, it is especially the emission in the solid angle region toward the front that is reduced to an impermissible extent. Moreover, as the frequency becomes higher, a pronounced lobe formation occurs in the diagram. Particularly at the minimum points of horizontal emission at a given emission output in the transmission mode, this often leads to undesirably low emission densities at the reception location, or in other words undesirably major radio field attenuation.
For actual practice, it is important in radio antennas that for a given transmitter output, the emission density not drop below a minimum required Value in any horizontal direction.